1. Field of the Invention
The present invention relates to a closed mixing machine for mixing a batch of polymeric materials such as rubber and plastic particles, and more particularly an improved mixing machine having non-intermeshed rotors which rotate at a predetermined rotation ratio and a predetermined phase angle of rotor tips for performing an improved macro-dispersion.
2. Description of the Prior Art
A batch type mixer is widely used for performing batch mixing of polymeric materials such as rubber and plastic particles, particularly in the tire manufacturing industry for preparing initial materials of the tires. Generally, there are two types of mixers used for those purposes; non-intermeshed rotor type and intermeshed rotor type.
FIG. 1(A) of the accompanying drawings shows a mixer of the former type in cross section. The mixer includes a casing 1 defining therein a pair of parallel mixing chambers 6,6 communicating with each other and closed at opposite ends thereof, and a pair of rotors 2,3 disposed in the chambers and spaced from each other. The casing 1 also has an inlet port 8 formed in the upper wall thereof at its center portion for introducing the material thereinto, in which port a ram 7 is disposed and closes the same. Each of the rotors has a number of helical rotor vanes extendng axially therearound. The rotors perform the mixing of a batch of the material charged thereinto via the port 8 as the rotors rotate. A mixer of an intermeshed rotor type shown in FIG. 1(B) has a construction similar to the mixer of FIG. 1(A) except in that a pair of rotors 2,3 are intermeshed with each other.
It is well known that the material undergoes basic processes consisting of macro-dispersion (distributive mixing) and micro-dispersion (shear mixing) during the batch mixing. Macro-dispersion is mainly caused by an axial drag flow created by the helical vanes of the rotors 2,3 rotating in the chamber and also by a transverse flow from one chamber to the other. On the other hand, micro-dispersion is mainly caused by a strong shearing force of the rotors 2,3 acting on the material moving transversely of the rotor axis. Due to the demand of users of the mixer, there are needs for reducing the mixing time for improvement of productivity, dispersing effectively the required additives for an improvement of quality in the mixing, and deterring a variance in quality for even or homogeneous mixing. In order to cope with those demands, various considerations have been taken into account for improving macro- and micro-dispersion. To this end, U.S. Pat. No. 3,403,894, British Patent No. 2024635A, Japanese Patent Application Nos. 53-76686, 53-76688, 54-61917, and 55-18335, for instance, disclose proposals for the shape, length and helix angle of the rotor vane, and the shape and diameter of the rotor and mixing chamber.
The mixer of FIG. 1(A) is advantageous in that since a space intercommunicating between the two chambers 6,6 beneath the ram 7 is wide enough to facilitate introduction of the material into the chambes, the shearing action occurs rapidly in comparison with the mixer of the intermeshed rotor type and thus achieves a high productivity. The mixer with the non-intermeshed rotors often operates at different rotor rotation speed, i.e., at a rotation ratio of 1.1 to 1.2, in which one of the rotors rotates faster than the other. The mixer of this type effects a rapid introduction of material and an increased degree of micro-dispersion when the two rotors 2,3 move into a synchronized mode where a phase angle of the rotor tips is 0 degrees as shown in FIG. 2(A). In this mode, the material 9 flows in a direction indicated by an arrow 10 at increased rate of material inclusion. In a non-synchronized mode as shown in FIG. 2(B), for instance, where the phase angle is 90 degrees, an increased degree of macro-dispersion is effected as the material 9 flows between the two chambers in an effective manner. To achieve a further improvement in productivity of this type of mixer, another modification has been made by increasing the number of rotor vanes (i.e. quadruple-vane) and /or rotor rotation speed. With those modifications, however, the variance in quality of the mixed material has been objectionably increased.
The inventor of the present invention has conducted research into the causes of the foregoing drawbacks by testing a simulation mixing machine I and a batch type mixer of 16 liters in capacity. The simulation machine I has mixing chambers having an inside diameter of 200 mm and a length of one third the length of normal chambers. Windows are provided in the wall of the chambers for observing the behavior of the material. One of the two rotors rotates faster than the other. As a result, it has been proved that a substantial variance in the temperature of the material occurs between the two chambers 6,6. The temperature of the material in one chamber in which the rotor rotates faster was higher by 10 degrees at maximum than that in the other chamber. The variance of the material temperature means that flow of the material from one chamber to the other takes place only insufficiently and hence the differential mixing energy, which is created by the two rotors rotating at different speeds in the two chambers, is not appropriately overcome or offset. In other words, the mixing quality of the material in the two chambers are not balanced. It has been also proved through analysis of the material in the chambers that a layer of the material 11, 12 sticks to the inner periphery of the chambers 6,6 except for an end face of the ram 7 and also on the surfaces of the rotors 2,3 as shown in FIG. 3. The material 11,12 which sticks have a viscosity which is different from that of the remaining material 9 (.+-.10 points at maximum of Mooney viscosity ). Sticking of the material takes place because the temperature of the material 9 is decreased enough to increase the viscocity and thus to deter the flow of the material when the latter comes in contact with the cooled surfaces of the metaric chambers 6. Obviously, the layers of sticking material 11,12 are replaced by the remaining material 9 only insuffciently. It is apparently understood from the research that the drawbacks of the known mixer will be overcome by further enhacing its macro-disperson, and more specifically by enhancing the replacing of the sticked material 11, 12 with the remaining material 9 and also inter-chamber flow of the material.